计算机代考程序代写 case study concurrency MAC Sub-Layer COMP90007 Internet Technologies – cscodehelp代写

MAC Sub-Layer COMP90007 Internet Technologies
Lecturer: Semester 2, 2021
© University of Melbourne 2021

Limited Contention Protocols
 Contention model + collision free model?
 Idea: divide stations into groups, within which only a
very small number are likely to transmit data.
 Avoid wastage due to idle periods and collisions
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Adaptive Tree Walk Protocol
 All stations compete for right to transmit, if a collision occurs, binary division is used to resolve contention
 Stations are divided into groups to poll
 Depth first search under nodes with poll collisions
 Start search at lower levels if >1 station want to transmit
groups
Example 1: D G
Slot 1D, G – collision Slot 2D
Slot 3G
Example 2: B D G
Slot 1B, D, G – collision Slot 2B, D – collision Slot 3B
Slot 4D
Slot 5G
stations
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Wireless LAN Protocols
 Wireless complications: stations have coverage regions, which leads to hidden and exposed terminal problems.
 When a station is in the range of two transmitters or relays, interference affects signal reception.
ADC B
 Require detection of transmissions around receiver,
not just carrier sensing.
 Transmission Protocols for Wireless LANs (802.11)
 Multiple Access with Collision Avoidance for Wireless (MACAW) 25

Hidden and Exposed Terminals (1)
 Hidden terminals are senders that cannot sense each other but nonetheless collide at intended receiver
 A and C are hidden terminals when sending to B  Want to prevent; loss of efficiency
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Hidden and Exposed Terminals (2)
 Exposed terminals are senders who can sense each other but still transmit safely (to different receivers)
 BA and CD are exposed terminals
 Desirably concurrency; improves performance
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MACA (1)
 MACA: Multiple Access with Collision Avoidance
 Sender asks receiver to transmit short control
frame
 Stations near receiver hear control frame
 Sender can then transmit data to receiver
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MACA (2)
MACA protocol grants access for A to send to B:
 A sends RTS to B [left]; B replies with CTS [right]  A can send with exposed but no hidden terminals
A sends RTS to B; C and B replies with CTS; D and E hear and defer for CTS E hear and defer for data
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Ethernet
 MAC Sub-Layer Case Study  Classic Ethernet
 Switched Ethernet
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Classic Ethernet
 Each type of Ethernet has a maximum cable length per segment.
 Multiple cable lengths can be connected by repeaters – a physical device which receives, amplifies and retransmits signals in both directions.
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Ethernet Frame Format
 MAC protocol is 1-persistent CSMA/CD
 Random delay (backoff) after collision is computed with BEB
(Binary Exponential Backoff, i.e., random number 0 to 2i − 1)  Frame format is still used with modern Ethernet
IEEE 802.3
• Preamble (7B) – synchronisation between sender and receiver
• Start of Frame (1B) – FLAG byte
• Dest. & Source addresses (6B + 6B) – to identify sender and receiver
• Type or Length (2B) – specifies which process to give the frame to
• Data (0~1500B)
• Pad(0~46B) – minimum size of an Ethernet frame is 64 Bytes
• CRC (4B) – 32 bits checksum
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Classic Ethernet Minimum Frame Size
 Collisions can occur and take as long as 2τ to detect  τ is the time it takes to propagate over the Ethernet
 Leads to minimum frame size for reliable detection
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MAC Addressing
 The MAC Address provides the unique identifier for a physical interface
 48-bit number encoded in the frame, written in hexadecimal notation
e.g. 00:02:2D:66:7C:2C
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Ethernet Performance
Channel Efficiency = 1+(2𝐵𝐵𝐵𝐵1𝐵𝐵)/(𝑐𝑐𝑐𝑐)
 F: frame length
 B: bandwidth
 L: cable length
 𝑐𝑐: speed of signal propagation; 𝑒𝑒: constant ≈ 2.71828  Optimal case: 𝑒𝑒 contention slots per frame
 When cF is large, the channel efficiency will be high.
 Increasing network bandwidth or distance (BL) reduces the efficiency for a given frame size.
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Switched Ethernet
 Hubs wire all lines into a single CSMA/CD domain
 Switches isolate each port to a separate domain  Much greater throughput for multiple ports
 No need for CSMA/CD with full-duplex lines
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Summary of Multiple Access Protocols
 Contention
 ALOHA, Slotted ALOHA
 Carrier Sense Multiple Access: 1-persistent, non- persistent, p-persistent
 Collision Free: bit map, binary countdown
 Limited Contention: adaptive tree walk
 MACA/MACAW (for Wireless LANs): RTS and CTS
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